Phosphine (PH.sub.3) is a preferred gaseous fumigant for stored grain because any residue of the fumigant will be lost or oxidised to a harmless phosphate when the grain is processed to produce a food. The phosphine is normally produced by the action of water or water vapour on a metal phosphide (typically aluminium phosphide or magnesium phosphide).
The conventional technique for generating phosphine for disinfesting a grain silo or the like involves the use of a probe to position formulations of aluminium phosphide or magnesium phosphide within the grain mass, where moisture from the grain, or in the air circulating through the grain, reacts with the phosphide to generate phosphine. A modification of this technique involves the placement of pellets of a metallic phosphide in the head-space of a grain silo and allowing the phosphine produced when the phosphide reacts with moisture in the atmosphere above the grain to be circulated through the stored grain by the natural convection currents that are present in the silo. These techniques, however, cannot be guaranteed to supply phosphine to all regions of the bulk stored grain to eradicate weevils and other unwanted grain pests. In addition, when these techniques are used, there is no control over the concentration of phosphine in the grain after the phosphide formulation has been added, and there is no possibility of regulating the concentration/time regime in the bulk storage to be most effective. The use of a probe to insert pellets into the grain mass also requires a significant labour input.
An alternative fumigation technique requires the placement of a quantity of a metallic phosphide (in granular or pellet form) in the head space above the grain, then actively recirculating air from the head space through the grain using recirculation ducts. This technique also has the disadvantage of lack of control over the concentration of phosphine in the grain after the phosphide formulation has been placed in the head space. The same disadvantage occurs when (in a modified form of this technique) the phosphide formulation is placed in a recirculation duct instead of being placed in the head space.
It has also been proposed that phosphine from cylinders of the gas (usually mixed with carbon dioxide) should be mixed with air externally of the stored grain and the resultant gas mixture should then be pumped through the grain. This disinfestation of the grain should be carried out periodically, or whenever it is perceived that fumigation of the grain is necessary or desirable. A major difficulty with this approach to the fumigation of grain is the need to guarantee the provision of expensive cylinders of the gas mixture (usually containing only 3 per cent phosphine) to the treatment site.
Clearly it would be advantageous to have a method of generating phosphine in a controlled manner at the treatment site using the readily available commercial phosphide formulations or similar light and easily handled feedstock. One such on-site phosphine generator is described in the specification of European patent application No 88119701.6, which is European publication No A-0318040.
There is one major problem with the use of phosphine, namely its flammability and explosivity in air at certain concentrations. The generally accepted maximum concentration of phosphine in air at standard temperature and pressure that can be used without the risk of an explosion is 1.79 per cent by volume. In a paper entitled "The Flammability Limit of Pure Phosphine-Air Mixtures at Atmospheric Pressure", by A R Green et al, published in "Controlled atmosphere and fumigation in grain storages" (Edited by B E Ripp et al), Amsterdam, Elsevier, 1983, pages 433-449, this "explosion limit" is confirmed and further information is provided about the flammability properties of phosphine. Other potential problems with the use of phosphine are the toxicity of phosphine to mammals when the phosphine is present in high concentrations, and the exothermic nature of the hydrolysis reaction to produce the phosphine. Thus the use of on-site phosphine generators, such as the generator described in European publication No. A-0318040, present a number of hazards to the user. In addition, with the fumigation regimes now being employed (for example, maintaining low phosphine concentrations for long periods), it is likely that the on-site phosphine generators may be left unattended for long periods. This is particularly likely to be the situation when the phosphine generator is used in connection with the fumigation of storages at remote locations.
Consequently, it is highly desirable that an on-site phosphine generator is used with a safety system which prevents the generation of phosphine in high concentrations in the event of a failure of the electrical power supply or any other system malfunction of the generator. It is also desirable to have a safety system which will prevent the build-up of phosphine within generators, due to residual and adsorbed moisture within the generator, following normal shut-down of the generator.
European publication No. A-0318040 discloses safety systems which admit an inert fluid into the phosphine generator, to displace moist air from the generator and thus prevent contact of that moist air with the metal phosphide in the generator, should there be a system failure leading to excessive phosphine concentrations. The safety systems are operable in response to a predetermined pressure drop within the generator. However, because these systems respond only to a loss of system pressure, they inherently provide a time delay before they become operable in the event of an electrical power failure. Furthermore, in the safety systems described in European publication No A-0318040, the purging fluid is preferably a liquid (for example, a mineral oil of low viscosity, methylene chloride or another, preferably organic, liquid which is inert to the metal phosphide contained in the generator). Thus a reservoir of the purging liquid must be provided at the generator site and this purging liquid reservoir must be full of the purging liquid at all times. Another disadvantage of using the purging fluids disclosed in European publication No A-0318040 is that they effectively destroy the phosphide bed in the generator and thus provide a safety system which can be used once only. There is also the problem of disposing of the contaminated bed of metallic phosphide following activation of the safety system, for the phosphide bed retains the potential to generate phosphine when it subsequently comes into contact with moisture.